1. 1. Physikalisches Institut, Universität Stuttgart, 70569 Stuttgart, Germanyde
2. Faculty of Civil Engineering, Transportation Engineering and Architecture, University of Maribor, SI-2000 Maribor, Slovenia
3. Experimentelle Physik VI, Universität Würzburg, 97074 Würzburg, Germany
4. Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, D-01328 Dresden, Germany